Reference redox couple

It is often necessary to compare the potentials obtained with one reference electrode to those obtained with another reference electrode. It is important that the electrodes in question use the same solvent and that the contribution from the liquid junction potential is minimized. Otherwise, we have to use the internal reference redox couple. The comparison can be done according to the diagram shown in... 

Quasireference electrode (QRE) — (-> reference electrode, pseudoreference electrode). An electrode that maintains a given, but generally not well-defined, potential during the course of a series of electrochemical experiments. It has the advantage of not contaminating the test solution by solvent or ions that a conventional reference electrode might contain and transfer. Thus in studies in aprotic solvents, like acetonitrile, a silver wire can behave as a QRE. It must be calibrated with respect to a true reference electrode or reference redox couple that is added at the end of the experiments to obtain meaningful potential values. 

In an electrochemical experiment the polymer is a film sandwiched between a metal electrode, the working electrode, and a liquid electrolyte (Fig. 1). Electrons can be exchanged between the polymer (2P) and the metal (it). The exchange proceeds across the polymer-metal interface (0V it). In addition, to maintain electroneutrality in the polymer bulk, ions are exchanged between and the electrolyte (%), across the polymer-electrolyte interface ( >/%). In the case of p doping, which corresponds to oxidation of the polymer, electrons are transferred from 0 to it (and the reverse for n doping, which corresponds to a reduction). The cell includes a reference electrode, which is immersed in the electrolyte and connected to a high-impedence voltmeter. It measures the potential of the electrolyte with respect to that of a reference redox couple. Note that no current... 

Thus the SECM is useful in probing heterogeneous kinetics at electrode surfaces, as well as other surfaces, like enzyme-containing membranes. The largest values of that can be measured are of the order of Did. Obviously this limit depends experimentally upon d, which is the characteristic length in SECM. Analogous measurements at a UME in bulk solution are similarly limited and provide a maximum on the order of DIa. For example, the for the ferrocene/ferricenium couple in MeCN, often used as a reference redox couple, as measured by SECM was 3.7 cm/s (22). 

In order to define a potential reference suitable for electrochemistry, a reference redox couple must firstly be chosen the H /H2 couple in its thermodynamic standard state. This reference system is called Standard Hydrogen Electrode (SHE). It is well-defined even though it is theoretical, and it is the reference used in all contemporary data tables in thermodynamics and electrochemistry... 

A well-defined redox couple can be used to calibrate an RE or as an internal standard in electrochemical experiments. The reference redox couple must be stable for the duration of the measurement, and must exhibit a repeatable potential in the system used. A good reference redox couple (63) for nonaqueous, and some carefully controlled aqueous systems, is the ferrocene/ferrocenium (FcIFc ) couple at 0.5-10.0 mM concentration. Standard reduction potentials, E°, for various solvents (64) are listed in Table 4.10. Other couples can be found in References (64-66). 

To circumvent the difficulty associated with the definition of the standard redox potential in the organic phase, a classical way is to refer aU the standard organic redox potentials to a reference redox couple such as ferrocinium/ferrocene. The standard redox potential of this reference couple in the organic phase is related to that in the aqueous phase by... 

In those instances where use of ferrocenes or derivatised ferrocenes is problematic due to non-ideal electrochemistry of the metallocenes or overlap of potentials with those of electroactive species making potential calibration problematic, the cobaltocenelcobaltocinium couple has been suggested as a reference redox couple [15]. The CcICc" couple is also an lUPAC recommended redox couple for electrochemical potential calibration [17]. Table 7.8 lists selected data for the CcICc couple in IL media. The cobaltocene electrochemical response is similar to that of the ferrocenes and the methodology of cahbration is again similar. Many researchers typically use the same voltametric techniques as discussed for the ferrocene and derivatised ferrocene couples to determine the Et value for the CcICc" couple. 

Lewandowski A, WaUgora L, GaUnski M (2009) Ferrocene as a reference redox couple for aprotic ionic liquids. Electroanalysis 21(20) 2221-2227. doi 10.1002/elan.200904669... 

Experimental redox potentials are typically reported with respect to a reference redox couple (RC), such as ferrocene/ferrocenimn ([FeCp2] ) ... 

Internal reference redox couples can provide results that are consistent over a wide range of experimental conditions since they bypass problems common to reference electrodes Like SCE or SHE that might generate liquid junction potentials. Even non-aqueous reference electrodes are prone to issues of reproducibility owing to electrode surface chemistry. These factors can contribute to discrepancies on the order of tens to hundreds of millivolts, even when comparing measurements reported under the same electrolyte and solvent conditions. Internal reference redox couples can remove these systematic uncertainties. 

The results of Section 1.5.3 show that a suitable choice of a reference redox couple is period dependent. For example, the [FeCp2] redox couple is a valuable reference for transition metal complexes of the first period in non-aqueous solutions and has been extensively used in a host of electrochemical studies under a wide variety of experimental conditions. In addition, several studies have explored first principle methods for calculating redox potentials with [FeCp2] as a reference RC. While comparisons for transition metal complexes of the first period have been successful, comparisons for second- and third-row transition metal complexes have proven to be more challenging. These discrepancies are likely to be due to systematic errors... 

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